Many players ponder this questions with very little guidance. The worst enemy of the underdeveloped player is clouded self observation. Self observation is fine to a point, but it is done so with the worst of prejudice, and fueled by our inner pride. I'm not saying that you should have someone looking over your shoulder constantly, I'm just saying that when we hit plateaus in our development and reach those hills that seem insurmountable, proper guidance and instruction from someone other than yourself is more helpful than trying the same thing over and over, expecting different results.
What should we practice though? This will be identified by examining your game and identifying your strengths and weaknesses. This should be done with a coach or instructor, and should be done over a period of time, say a week or two. Paying for one hour of instruction and having an instructor watch you get into your stance (cold out of the seat and under no pressure) is completely different than having a coach or instructor evaluate your performance during competition. It's apples and oranges. I evaluate my students during tournaments and league play, because the atmosphere of competition will highlight any weaknesses that may exist -either above or below the surface. When weaknesses have been detected and identified, we can then evaluate them and see if the visible, above the surface weakness has roots beneath the surface, therefore creating a group of sub-weaknesses - usually cloaked as bad habits. It is only then that we can set up a practice routine that is designed to attack these deficincies in your game and turn your weaknesses into strengths.
Is this done by drills &amp; patterns? Sometimes. Drills and patterns are necessary. Yet tedious and boring, they are vitally important to developing as a player in much the same way scales are important to the development of a pianist, or flight instruction to an airline pilot. They add a sense of discipline to your practice routine. If self observation is the number one enemy, lack of discipline runs a close second. There are many players who have excellent natural ability to play the game of pool, yet they lack the discipline to bring their game to its full potential. While this is a sad fact, understand that it is factors such as this that separates the champions from the rest of the field.
So we have detected a weakness and now it is time to dedicate practice time to the task at hand, which is to DEVELOP the weakness into a strength. This is what practice is about. If your practice is dedicated to any other purpose, you are tossing snowballs into a furnace. Many of us suffer in this area for one reason:
Our approach
So how do we approach practice? I can only speak from my experience as a former touring professional. My approach was simple. I was a pool player. That is what I do. My goal was to be the best player that I could possibly be. While striving for progress rather than perfection, I charted my progress diligently, ironing out the wrinkles as I found them. It is amazing how new areas of weakness are exposed to you at every turn. My approach became very simple. I wanted to leave the table a better player than when I arrived. This meant filtering out a few bad habits, as well as people. Practice was serious business, yet entertaining and challenging, ever-developing and expanding, and ever-changing.
Variety seemed to be the key. As my game became stronger, I started to notice something. Every flaw that was detected in my game was rooted in the foundation of the basic fundamentals of stroke, stance, sighting, weight distribution, balance, breathing,and follow through. These flaws worked their way up tp the surface where they affected my position, shooting accuracy, shot selection, bank shots, safety play and speed control. The deficiencies would therefore alter my confidence level, leading to problems during competition that stemmed from nerves, and lack of emotional &amp; self control. Armed with this knowledge, I was able chart the patterns of the weaknesses. I could trace an emotional outburst caused by missing a shot back to a flaw in my stance. To attack the problem, I had to view and recognize the weakness. This is the main flaw in self-observation. Pride rears its head in and tells us that is was the table conditions, or the railbirds that caused us to miss the shot. We place the blame externally (naturally) instead of internally. Acceptance of the problem is key. An open mind is required. Then we must establish goals related to overcoming the weakness. Goals should be short term, mid term and long term, and they MUST have deadlines. We then prepare a practice routine designed to attack the problem. You must make a commitment to overcome the weakness. This also requires determination. The probelm will not disappear overnight, nor will it disappear with a few practice sessions. Succeeding depends on having the proper attitude while armed with the right information. Knowing where to start is vital. Rewarding yourself for your accomplishments is vital as well.
The purpose of practice is three fold:
Identify weaknesses
Develop weaknesses into strengths
Strengthen the skills you already possess

We can also test out new ideas try new things during practice. Weaknesses can be detected during competition as well as they can be detected during practice. Weaknesses usually unveil themsleves at the worst possible opportunities. I look at it like this: A weakness displayed at the worst time makes it more memorable, and I am more likely to deal with it sooner rather than later. It's the mistakes that are "shrugged off" that usually develop into worse habits and weaknesses. Weaknesses are just like strengths. They grow bigger and spread like a cancer to other parts of your game.
Now to dispell something that is a problem with development. Don't fall into the trap of looking at practice as a chore. It should be disciplined, but not boring and tedious. This is why practice "routines" are simply that.... "routines". Routines fade away and are abandoned. They also lead to ruts. I once heard if you gave a pool player a rut he'd probably move in and furnish it. Practice should be part of your daily routine. They need not be 6 hour marathons. An hour a day focusing in on one specific area of your game can and will do wonders for you. Practice should be a way of life, and should not be what I call "programatic". Myself, I follow training cycles followed by periods where I completely cut myself off from the game to avoid burnout. This works for me, it may not work for you.
Above all else, you must love what you are doing. If you are not enjoying yourslef, why bother? Pool is a fun game and it is also a game where you can learn something new and fun every single time you play. I grade my trips to the pool hall NOT by how much I won or lost, but by how much fun I had while I was there. That is what this game is all about.